Generally, in a plasma processing apparatus, plasma of a processing gas is generated within a decompression processing vessel. Further, a thin film is formed on a processing target object within the processing vessel by a gas phase reaction or a surface reaction of radicals or ions included in the generated plasma, or micro-processing such as etching of a material or a thin film on a surface of the processing target object is performed.
A capacitively coupled plasma processing apparatus includes an upper electrode and a lower electrode arranged in parallel to each other within a processing vessel. A processing target object (e.g., a semiconductor wafer, a glass substrate, etc.) is mounted on the lower electrode, and a high frequency power having a frequency (typically, 13.56 MHz or higher) suitable for plasma generation is applied to the upper electrode or the lower electrode. Electrons are accelerated in a high frequency electric field generated between the upper electrode and the lower electrode by applying the high frequency power, and plasma is generated as a result of ionization by collision between the electrons and a processing gas. Further, a RF bias method is widely employed. In this RF bias method, a high frequency power having a relatively low frequency (typically, 13.56 MHz or lower) is applied to the lower electrode on which the processing target object is mounted, and ions in plasma are accelerated and attracted to the processing target object by a negative bias voltage or a sheath voltage generated on the lower electrode. According to this RF bias method, by accelerating the ions in the plasma and bringing them into collision with the surface of the processing target object, a surface reaction, an anisotropic etching or a film modification may be facilitated.
Recently, for the purpose of improving a yield or a processing accuracy of dry-etching, for example, in order to suppress a charging damage (a destroy of a gate oxide film by accumulation of electric charges) or a micro-loading effect (non-uniformity of an etching rate caused by a local difference in a pattern density or a geometrical structure of a pattern), a technique of pulse-modulating a high frequency power for plasma generation and/or a high frequency bias power with a pulse having a preset frequency is widely used.
Generally, in this type of pulse modulation, according to a duty ratio of a modulation pulse, a high frequency power to be pulse-modulated is controlled to be in an on-state of a preset level during a pulse-on period, and controlled to be in an off-state of a zero level during a pulse-off period. Accordingly, when pulse-modulating the high frequency power for plasma generation, for example, plasma is generated and etching progresses during the pulse-on period, whereas the plasma is extinguished and the etching stops temporarily during the pulse-off period. In this case, a matching device provided on a transmission line for supplying the high frequency power for plasma generation measures a load impedance during a pulse-on period within each cycle, and controls a reactance of a variable reactance element provided in a matching circuit such that the load impedance measurement value may be equal to or approximate to a matching point (typically, 50Ω).    Patent Document 1: Japanese Patent Laid-open Publication No. 2012-009544    Patent Document 2: Japanese Patent Laid-open Publication No. 2013-033856
As one example of the above-described pulse modulation in the capacitively coupled plasma processing apparatus, there is known a method of controlling the high frequency power to have a preset high level during the pulse-on period and controlling the high frequency power to have a preset low level lower than the high level during the pulse-off period. Here, the low level is set to be a value higher than the lowest level required to maintain the plasma generation state.
In this high/low pulse modulation method, even during the pulse-off period, certain amounts of electrons, ions, and, also, radicals exist within the processing vessel without being completely extinguished. By using them, by setting the low level of the corresponding high frequency power and other process parameters to appropriate values, chemical or physical actions of the electrons, the ions and/or the radicals on the surface of the processing target object can be controlled, so that a preset etching characteristic is expected to be improved in a certain kind of etching process.
In this high/low pulse modulation method, however, if the frequency of the modulation pulse is set to a high value (typically, 1 kHz or higher), control of the variable reactance element provided in the matching device cannot follow up the modulation pulse. For this reason, a matching operation is performed only during a pulse-high period which dominantly contributes to the plasma process, and the matching operation is not performed during a pulse-low period. If so, during the pulse-low period when the matching operation cannot be performed at all, a large reflection wave is generated on a high frequency transmission line. As a result, it may be difficult to control the high frequency power to be maintained at the preset low level stably and accurately, and, besides, an expected effect of the high/low pulse modulation method on the process may be weakened. Further, a load on a high frequency power supply or the like may be increased.